Structure for forming a reaction product

ABSTRACT

A reaction product is formed by a process which involves the transfer of the reaction product from the autoclave to a receiving vessel at a substantially constant flow rate. Just prior to this transfer, the pressure in the receiving vessel is brought up to the pressure in the autoclave by passing gas from the autoclave to the receiving vessel. The flow of gas from the autoclave to the receiving vessel is then stopped, and the pressure in the receiving vessel is allowed to drop due to transfer of heat from the gas to the walls of the receiving vessel. The resulting pressure difference between the autoclave and the receiving vessel is used to initiate the transfer of the reaction products from the autoclave to the receiving vessel. A pressure release valve on the receiving vessel is then controlled by means a signal derived from a flow meter which measures the flow rate of the reaction products flowing from the autoclave to the receiving vessel to maintain constant this flow rate.

This application is a division of application Ser. No. 06/887,753 filedJuly 18, 1986, now U.S. Pat. No. 4,753,787.

FIELD OF THE INVENTION

This invention relates to a method of forming a reaction product such ascalcium silicate, titanium calcium oxide, magnesium calcium oxide, andzirconium calcium oxide and similar reaction products and the structureemployed to form these reaction products.

DESCRIPTION OF THE PRIOR ART

In my earlier U.S. Pat. Nos. 4,238,240, 4,366,121 and 4,545,970 Idescribe numerous prior art structures and processes for formingreaction products. These prior patents are hereby incorporated byreference.

In my '240 patent I disclose a method for forming a reaction product inwhich the reaction constituents are mixed in an autoclave, the mixedreaction constituents are then reacted for a selected time to formreaction products and the reaction products are transferred, at the endof the reaction, from the autoclave to another vessel (sometimes calleda "receiving vessel" and sometimes called an "antipressure vessel")connected to the autoclave by a flow passage. The pressure in the vesselis held in a controlled manner beneath the pressure in the autoclaveduring the transfer of the reaction products from the autoclave to thevessel. To maintain the pressure in the vessel in a controlled mannerbeneath the pressure in the autoclave during the transfer of thereaction products from the autoclave to the vessel, I disclose anelectronic control system which measures the pressures in the autoclaveand the vessel and which opens or closes a valve (shown as valve 101 inFIG. 1 of the '240 patent) attached to the receiving vessel (vessel 12in the '240 patent) to maintain the pressure in the vessel beneath thepressure in the autoclave (shown as autoclave 10 in the '240 patent). Ialso disclose an alternative embodiment in the '240 patent wherein theelectronic control system is replaced by a throttle valve or by a valveand a vent pipe. Before the start of the transfer operation, a suitablepressure difference is established between the autoclave and thereceiving vessel. Then to start the transfer of the reaction productfrom the autoclave to the antipressure vessel, a valve between theautoclave and the vessel is opened and simultaneously or subsequently,as desired, a pressure release valve on the top of the receiving vesselis opened and left open during the transfer process. As a result, thereaction product from the autoclave flows into the vessel at aninstantaneous rate determined by the instantaneous pressure differencebetween the autoclave and the receiving vessel. As I disclose in the'240 patent, this pressure difference is controlled by the sizes of thevalve and vent pipe or the setting of the throttle valve. Thisembodiment avoids the use of a control circuit but has the potentialdisadvantage that the transfer is not as precisely controlled as with acontrol circuit.

SUMMARY

In accordance with this invention I provide a substantially simplifiedsystem for transferring the contents of the autoclave 10 to theantipressure vessel 12. The system of this invention incorporates apressure release valve on antipressure vessel 12, the setting of whichis precisely controlled by a control signal from a flow meter used tomeasure the volumetric flow of the reaction product. In the preferredembodiment the pressure release valve is controlled to maintain aconstant flow of reaction product from autoclave 10 to antipressurevessel 12.

My invention provides a novel method of initializing the pressure in theantipressure vessel 12 by releasing gas (typically steam) from theautoclave 10 through a vent pipe into the antipressure vessel 12 priorto the transfer of reaction product from the autoclave to theantipressure vessel 12. When the pressure in vessel 12 is equal to thepressure in autoclave 10, the vent pipe is closed and the pressure invessel 12 falls slightly beneath the pressure in autoclave 10 as aresult of the natural cooling of the gas in vessel 12 due to heattransfer to the relatively cooler walls of vessel 12. As vessel 12 comesto a relatively steady state temperature after several batches ofreaction product have been passed to vessel 12, the pressure differencebetween autoclave 10 and vessel 12 due to this natural cooling effectbecomes less and when the gas is steam, relatively little steamcondenses to create this pressure difference. This method and structureavoids the use of costly compressors as in the prior art to initializethe pressure in antipressure vessel 12. When the gas is steam, themethod requires a surprisingly small amount of steam from the autoclave10 to pressurize the antipressure vessel 12 due to the fact that thesteam in the autoclave 10 is at a high pressure and temperature andtherefore contains a high volume of H₂ O per cubic meter.

This invention will be more fully understood in conjunction with thefollowing detailed description taken together with the attached drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an autoclave and an antipressure vesselinterconnected in accordance with the principles of this invention.

FIG. 2 illustrates schematically the control system of this invention.

DETAILED DESCRIPTION

The following detailed description is intended to be illustrative onlyof one embodiment of the invention and not to limit the invention. Thespecification of my U.S. Pat. No. 4,238,240 illustrates in detail onereaction process for the formation of calcium silicate and will bereferred to from time-to-time in the following description.

As will be apparent from a comparison of FIG. 1 with FIG. 1 of the '240patent, the system of this invention for the formation of a reactionproduct is substantially changed from that disclosed in the '240 patent.Thus autoclave 10 possesses an outlet controlled by valve 10c and aninlet controlled by valve 10d both of a type well known in the art Anagitator 10a has a plurality of paddles 100a, 100b to 100i where i is aninteger equal to the maximum number of paddles used with the agitator.The blades on the paddles are preferably of the INTERMIG.sup.® typesupplied by EKATO in West Germany. In the preferred embodiment of thisinvention six paddles are used on agitator 10a. However, a differentnumber of paddles can be used if desired based upon experimentalresults. Agitator 10a is, in accordance with this invention, a variablespeed agitator with a speed which, in one embodiment, varies from 60 rpmto 150 rpm. Of course, these speeds can also be changed if desired toachieve appropriate results depending upon the reaction productsdesired.

Autoclave 10 is heated by the use of a thermal oil of well knownconstituents. The thermal oil first is heated in a thermal oil boiler(not shown but well known in the arts) and then is pumped through hollowsemicircular coils wound in a plurality of banks on the outer surface ofautoclave 10. FIG. 1 shows eight cross-sections 15a through 15h of onebank of such semicircular coils. Typically four banks of coils are usedand one bank contains eight (8) spirals of heating coils which pass thethermal oil in one direction. The adjacent bank also contains eight (8)spirals of heating coils but passes thermal oil in the other direction.The use of the plurality of banks of coils minimizes the temperaturedrop of the heating oil in any one bank to ensure that the surface ofthe autoclave is reasonably uniformly heated in the steady state. In oneembodiment, the temperature drop of the heating oil from the inlet tothe outlet of the bank is kept to less than twenty degrees centigrade.This small temperature drop coupled with the use of the agitators allowsthe temperature of the reaction product in the autoclave to be keptsubstantially uniform within about ±1° C.

Agitator 10a within autoclave 10 is controlled to mix the reactionproducts within autoclave 10 to ensure substantially uniform temperaturethroughout the reaction products. Properly controlling the speed ofagitator 10, gives substantially uniform temperature throughout theautoclave. As a result, the crystal growth of the reaction productwithin autoclave 10 is also controlled to be substantially uniform.

The reaction product is formed by controlling the temperature of thereaction constituents within autoclave 10 to within a selected value fora selected period of time at a desired pressure. In one embodiment, thepressure in autoclave 10 is increased substantially over that disclosedin my prior U.S. Pat. No. 4,238,240. In particular, in my preferredembodiment for forming calcium silicate reaction products in accordancewith this invention, the temperature within the autoclave 10 is held at318° centigrade, corresponding roughly to 110 BAR pressure absolutewhich is the saturation pressure of the water with the reactionconstituents. The use of this high temperature and pressure has severalbeneficial effects. First, when one is fabricating a reaction productsuch as calcium silicate, a reaction constituent such as silica is moresoluble in water at 318° centigrade than at a lower temperature. Thismeans that a lower quality silica can be used in the reaction withoutdetrimentally affecting the quality of the reaction product and that thereaction will take place at a higher rate within the autoclave. Asdisclosed in a report published by Lawrence Berkeley Laboratory,University of California (LBL-14722) entitled "A Database for NuclearWaste Disposal for Temperatures up to 300° C." by Sidney L. Phillips andLeonard F. Silvester, Sept. 1982, the amount of silica in solution canbe calculated according to equation 15 set forth in that paper. Thisequation states that log S (where S is the solubility of silica in waterin gram moles per liter) is a function of temperature as follows:

    Log S=-0.320-697.12/T(K)                                   (1)

Using this equation one can calculate the solubility per liter ofamorphous silica SiO₂ dissolved in water at the preferred temperature.As the temperature goes up, the amount of material dissolved also goesup. Similar equations are available for other inorganic materials givingthe solubility of these materials in water as a function of temperature.Accordingly, there is a substantial advantage to operating the autoclave10 at a higher temperature and pressure than previously consideredadvisable.

The system in FIG. 1 also includes an outlet valve 10c connected to anoutlet line 17 (composed of sections 17a, 17b and 17c). Outlet line 17passes the reaction products from autoclave 10 through heat exchanger11. Heat exchanger 11 contains an inlet 11a and an outlet 11b for thepassage of a fluid into the heat exchanger 11 to receive the heat fromthe reaction product flowing through line 17. As described in the '240patent, the removal of heat from the reaction products makes availablethe transferred heat for further use thereby increasing the efficiencyof the process and further stabilizes the reaction products in a desiredform before the reaction products reach the antipressure vessel 12.

The reaction products flowing through pipe 17 pass into heat exchanger11 at inlet 11c and out from heat exchanger 11 through outlet 11d. Fromoutlet 11d the reaction products flow through pipe 17b, flow meter 13(preferably a magnetic flow meter), pipe 17c and inlet valve 12c intoreceiving vessel 12. Vessel 12 likewise contains an agitator 12acontaining a plurality of paddles 120a, 120b...through 120i, where i islikewise an integer representing the number of paddles on agitator 12a.The blades of paddles 120 are also preferably the INTERMIG.sup.® typefrom EKATO in West Germany. One embodiment of this invention uses six(6) such paddles although again the number of paddles used can bedetermined empirically depending upon the quality desired for theresulting product. Vessel 12 has an outlet 12e with a valve 12f forcontrolling the removal of material from vessel 12. In addition, a vent12d is provided. Vent 12d includes a pressure release valve (preferablya digital valve) which is electronically coupled to flow meter 13 insuch a way as to maintain substantially constant the volumetric flow ofmaterial from autoclave 10 into holding vessel 12. This ensures that thesteam from autoclave 10 which was previously placed in receiving vessel12 prior to the start of the transfer of the reaction products fromautoclave 10 to vessel 12 is released from vessel 12 at the samevolumetric rate as the reaction products from autoclave 10 enter vessel12.

Prior to the transfer of reaction products from autoclave 10 to vessel12, steam from autoclave 10 is bled from autoclave 10 into vessel 12 byopening valves 10b and 12b on line 14 connecting autoclave 10 to vessel12. Steam is allowed to flow from autoclave 10 to vessel 12 until thepressure in vessel 12 equals the pressure in autoclave 10. As soon asthe pressures are equal in vessel 12 and autoclave 10, valves 10b and12b are closed and valves 10c and 12c are opened. Because vessel 12 hasnot been heated by thermal fluid as has vessel 10, the natural transferof heat from the steam within vessel 12 to the walls of vessel 12 coolsdown the steam and lowers the pressure within vessel 12, therebystarting the transfer of the reaction products from autoclave 10 tovessel 12.

As soon as the flow of the reaction products from autoclave 10 to vessel12 starts, valve 12d (preferably a digital valve) is opened to aselected value to maintain the flow as detected by magnetic flow meter13 at a selected value. Alternatively, valve 12d can be opened to lowerthe pressure in vessel 12 and start the transfer of the reactionproducts from autoclave 10 to vessel 12. Flow meter 13 measuresvolumetric flow rate. Of course, a mass flow meter can be used ifdesired. Flow meter 13 produces an electrical output signalrepresentative of the volumetric flow rate of the reaction product fromautoclave 10 to vessel 12. This electrical output signal is transmittedto control 16 of a well-known design which in turn produces a digitaloutput signal which controls the setting of digital valve 12d. Shouldthe flow rate of reaction products through flow meter 13 be beneath thedesired value, valve 12d is opened to decrease the pressure in receivingvessel 12 by allowing steam within that vessel to vent to theatmosphere. Should the flow rate of reaction products through magneticflow meter 13 be higher than desired, control 16 closes down valve 12dto reduce the amount of steam allowed to escape from vessel 12 therebyto properly control the flow rate of reaction products through magneticflow meter 13 to the desired value. Flow meter 13 typically has anaccuracy of about plus or minus 1% over its whole range and thus theflow of reaction products through pipe 15 can be controlled within thisaccuracy using a negative feedback control system. In this system theoutput signal from the flow meter 13 is compared to a reference signalrepresentative of the desired volumetric flow rate of the reactionproducts from autoclave 10 to vessel 12 and the difference between thesetwo signals, expressed as a digital signal, is used to control thesetting of valve 12d. While the volumetric flow rate is controlled inthe preferred embodiment, mass flow rate could, if desired, becontrolled. As part of the control system, the instantaneous pressuresin autoclave 10 and vessel 12 are measured using sputtered film pressuretransducers of a type made available by CEC Corporation in Pasadena,California. These transducers are linear and reproducable over a rangeof pressures typically up to several hundred atmospheres and retaintheir accuracy over their lifetime.

FIG. 2 illustrates schematically the control system of this invention.The magnetic flow meter 13 comprises several components. A flow detector130 actually detects the volumetric flow Q_(RP) of reaction product fromautoclave 10 to vessel 12 and produces an electrical output signale_(fd) which is transmitted to the noninverting input lead of adifferential amplifier 131. A reference voltage e_(ref) from a referencevoltage source 132 is applied to the inverting input of amplifier 132.The output voltage e_(out) from differential amplifier 131 representsthe difference in the output signal e_(fd) from the flow detector 130and the output signal e_(ref) from the reference source 131. This outputvoltage e_(out) is transmitted to valve control unit 16 which generatesa digital output signal e_(v) (which comprises six bits transmitted todigital valve 12d preferably in parallel on a six channel bus) whichcontrols digital valve 12d mounted on vessel 12 (see FIG. 1). Digitalvalve 12d is selected because it is highly linear and does not exhibitsignificant hysteresis. Moreover, digital valve 12d can be adjusted veryrapidly to any one of 64 possible different settings withinmilliseconds. Typically, digital valve 12d has 6 ports each sizeddifferently to handle a different flow. The combination of all 6 portsopen gives the maximum flow through the valve whereas leaving open onlythe smallest port gives the smallest flow. Each port is controlled byits own magnetic coil and thus the valve can be driven to any one of 2⁶or 64 linearly related positions extremely rapidly.

The output from valve 12d is the flow rate of gas Q_(G12) from vessel12. This flow rate reduces the pressure in vessel 12 as shown in FIG. 2by the negative sign on the input of the arrow from valve 12d to theblock 22 labeled "pressure in vessel 12." The pressure in vessel 12 isalso increased by the flow of reaction products from autoclave 10 intovessel 12 as illustrated by the plus sign on the arrow associated withthe line relating to the flow rate Q_(RP) of the reaction products fromthe autoclave 10. The pressure in vessel 12 generally restrains the flowof reaction products from autoclave 10 to vessel 12 and thus the outputfrom the block 22 labeled "pressure in vessel 12" is shown as P12 andgiven a negative sign as an input to line 17. This indicates that thispressure acts as a back pressure on the flow of reaction productsthrough line 17. On the other hand, the pressure P₁₀ in vessel 10 drivesthe reaction products from autoclave 10 to vessel 12 and thus is shownas a positive influence on line 17. The output from box 24 labeled line17 is the flow rate of reaction products Q_(RP) from autoclave 10 tovessel 12. The flow of reaction products from autoclave 10 decreases thepressure in autoclave 10 and this is shown by the negative arrow labeledQ_(RP) pointing to block 23. The flow detector 130 detects the flow rateQ_(RP) of the reaction product from the autoclave 10 to vessel 12 andproduces the output signal e_(out) representing this flow.

In operation, should the flow drop beneath the desired flow rateindicated by e_(ref), e_(fd) drops beneath e_(ref) and the output signale_(out) becomes positive thereby driving valve control unit 16 to openvalve 12d. Opening valve 12d increases Q_(G12) thereby dropping furtherthe pressure P12 in vessel 12. This increases Q_(RP) Increasing Q_(RP)drops further the pressure P₁₀ in autoclave 10 and increases thepressure P₁₂ in vessel 12. However the flow Q_(G12) is properly selectedto increase Q_(RP) to the desired value. On the other hand, shoulde_(fd) be larger than e_(ref), e_(out) is negative and thus reduces thevoltage e_(v) used to control the setting of valve 12d therebydecreasing Q_(G12) and increasing slightly the pressure P₁₂ in vessel 12over its nominal value for that time. This slows down the flow ofreaction products from autoclave 10 to vessel 12 thus decreasing Q_(RP)

In the preferred embodiment, the reaction products are transferredthrough pipe 17 under laminar flow conditions thereby preventing thecrystal structure formed in autoclave 10 from degrading. For safety'ssake, the same pressure transducers placed on the top of autoclave 10and vessel 12 are also connected to safety control circuits to preventthe inadvertent opening by individuals operating the system of anyvalves during the reaction process In addition, safety valves are placedon the top of autoclave 10 and the receiving vessel 12 to relievepressures within these vessels should these pressures exceed safetylimits.

The process and structure described above is multi-purpose in the sensethat the process and structure can be used to provide a number ofdifferent reaction products. The pressure and temperature of the processdescribed above have increased substantially compared to the processdescribed in my '240 patent. The process is especially suited for theproduction of new, higher resistance insulation materials such ascomposites formed of magnesium, zirconium, and titanium among othermaterials. In addition, the process can be used to produce ceramicpowders such as silicon carbide, silicon nitride, and titanium diborideby means of hydrothermal reactions. This is made possible by the hightemperature and pressure used in the reaction process of this invention.

Typical reactions which can be carried out by the structure and processof this invention are those to form titanium calcium oxide, magnesiumcalcium oxide or zirconium calcium oxide, as follows: ##STR1## The abovereactions are carried out at 318° C. and 110 BAR pressure absolute,which correspond to saturated steam temperature and pressure. Thereaction time is selected as a function of the crystal size required.The above hydrothermal reactions are endothermal.

The use of a hydrothermal reaction of this invention to form ceramicpowders saves a substantial amount of energy over standard methods forthe formation of such ceramic powders. Moreover, the hydrothermalreaction provides ceramic material of substantially uniform crystal sizein a powder like form. A typical prior art process for forming suchpowders involved melting ingredients at a very high temperature (2800°C.-3000° C.), allowing the melted ingredients to cool in a large blockto ambient temperatures, crushing the block into smaller parts, coarselygrinding the smaller parts to yield rough crystals and then finelygrinding the rough crystals to yield fine powders. By using myinvention, this energy intensive process is totally avoided. Myhydrothermal process will produce directly fine crystal powder. Thehydrothermal reaction takes place at a temperature in the 300° C. rangerather than at several thousand degrees centigrade. By controlling thetime of reaction the size of the ceramic crystals can be fairlyaccurately controlled to the desired dimension. Thus the processdescribed above yields a substantial improvement in the formation ofuniform crystals of reaction products over the prior art both in termsof energy consumed and the uniformity of the resulting structure.

In addition, the prior art grinding procedure yields crystals ofnonuniform and differing sizes even though the resulting materials aresubstantially fine. This creates certain problems in using thesecrystals to form finished products. In particular, ceramic materials areknown to be brittle despite their other desirable characteristics.Because of this shortcoming, ceramic materials find fewer applicationsin advanced technology than justified by their potential benefits. Thusresearch is being done to increase the lifetime and prolong the fatiguelimits of ceramic materials such that ceramic materials can be used innew applications to replace a variety of metal composites. However,nonuniformity of ceramic crystal size yields a nonuniform bonding forcewhich in itself relates to discrepancies in the atomic structure of theceramic crystal making up the ceramic materials. Scanning electronmicroscope (SEM) exposures of ceramic materials show that fatigue startsat those places where there are substantial differences in uniformity ofthe ceramic crystals. Apparently the bonding energy between nonuniformcrystals is unable to find a so-called harmonic neighbor thus leading tospontaneous fatigue because of the differences in the bonding energybetween different size crystals within the material. At this stage ofthe technological development of materials from ceramic crystals,several companies have acquired improved crystal size uniformityobtained using a grinding process but still the uniformity is notsufficient to allow the proven material to be used in high technologyapplications such as blades for jet engines. Thus considering thesefactors, the process of my invention makes possible the fabrication ofuniform powders.

My process has the following characteristics:

1. Controlled temperature of the reaction process within plus or minusabout one (1) degree Kelvin;

2. Control of pressure by use of pressure transducers such as sputteredfilm transducers of a type made available by CEC Corporation inPasadena, Calif.

3. Variable speed stirring equipment using INTERMIG.sup.® blades of atype provided by EKATO in West Germany;

4. Reproducibility of reaction products as a function of reaction timeand temperature;

5. Use of less energy than prior art processes;

6. Plurality of different reaction products capable of being made withthe same system; and

7. The attaining of higher precalculable solubility for reactionconstituents to allow accurate characterization of the process.

In view of the above, other embodiments of this invention will beobvious to those skilled in the art.

What is claimed is:
 1. A system for forming reaction productscomprising:an autoclave for reacting reaction constituents for aselected time and pressure to form non-gaseous reaction products and gasunder pressure; a vessel adapted for receiving said non-gaseous reactionproducts; a pressure release valve on said vessel; a flow passageconnecting said autoclave to said vessel, said flow passage beingadapted for the transfer of said non-gaseous reaction products from saidautoclave to said vessel; means for transferring said gas under pressurefrom said autoclave to said vessel prior to the transfer of saidnon-gaseous reaction products and after a reaction process issubstantially completed in said autoclave; and means for controlling asa function of time the pressure in said vessel to be beneath thepressure in said autoclave so as to maintain constant the flow rate ofsaid non-gaseous reaction products from said autoclave to said vessel.2. The system as in claim 1 wherein said means for transferring gas fromsaid autoclave to said vessel includes;a gas flow passage connecting atop of said autoclave to a top of said vessel; and valve means connectedin said gas flow passage to either allow or prevent, as desired, theflow of gas from said autoclave to said vessel.
 3. The system as inclaim 1 wherein said means for maintaining constant the flow rate ofsaid non-gaseous reaction products from said autoclave to said vesselcomprises:means for measuring the flow rate of the non-gaseous reactionproducts flowing from said autoclave to said vessel; means forgenerating an electrical signal proportional to the difference betweensaid flow rate and the desired flow rate; means, responsive to saidsignal from said means for generating an electrical signal, forcontrolling the setting of said pressure release valve said pressurerelease valve controlling the rate at which gas is released from saidvessel, to yield a substantially constant flow rate of said non-gaseousreaction products from said autoclave to said vessel.
 4. The system asin claim 3 wherein said pressure release valve comprises a digitalvalve.
 5. The system as in claim 3 wherein said means for generating anelectrical signal comprises a magnetic flow meter.